A conventional flow cell and particle analyzer is shown, in plan view, in FIG. 1, and a particle analyzer is shown in block schematic form in FIG. 2. The flow cell and particle analyzer automate the job of counting fluid particles suspended in a biological sample (for example, blood cells suspended in a serum sample or particles suspended in a urine sample). More generally, the flow cell and particle analyzer may be used to automate the job of counting any "particles of interest" that are suspended in a fluid sample.
Referring now to FIG. 1, the flow cell includes a body 10 containing a flow chamber having an inlet 12 through which the fluid sample is provided. The flow chamber also has an outlet 14. A passageway 16 extends between the inlet 12 and the outlet 14 such that the fluid sample provided at the inlet 12 flows past an examination area 18 (shown by a dashed line in FIG. 1) and is discharged out of the passageway 16 via the outlet 14.
During a sampling run, the fluid sample is surrounded on both sides by sheath fluid (e.g., saline solution) introduced into the passageway 16 from inlets 13a and 13b. A strobe light 32 (FIG. 2) illuminates the examination area 180 from below and a microscope 30 is focused on the examination area 18 from above. A CCD camera 34 receives the output of the microscope 30 to form a series of still frame images.
In particular, a frame grabber 40 "grabs" the still frame images and stores them in a memory. A CPU 56 combines selected components of the still frame images to generate composite images which represent the particles in multiple images. Thus, individual particles (for example, different types of blood cells) can be analyzed and sorted.
In order for the particle analyzer to perform reliably, the fluid sample having the suspended particles of interest must flow uniformly through the flow cell examination area 18, without interruption. Even if interruptions occur relatively infrequently, when they do occur they will cause the determined particle counts to be inaccurate.